For example, JP4569371B2 (corresponding to US2006/0243938A1) recites a linear solenoid, which includes a plunger made of a magnetic material, a yoke made of a magnetic material and a stator core. The stator core includes a magnetically attracting core, a slide core and a magnetic shield portion, which are formed integrally.
In this linear solenoid, the plunger is placed on a radially inner side of a coil and is movable in an axial direction. The yoke is configured into a cup form and includes an opening, a bottom wall portion and a peripheral wall portion. The peripheral wall portion covers an outer peripheral portion of the coil, and the bottom wall portion covers one axial end of the coil.
The magnetically attracting core of the stator core is made of a magnetic material and magnetically attracts the plunger toward the other axial side, which is opposite from the one axial end of the coil, with a magnetic flux generated through energization of the coil. The slide core is made of the magnetic material and is configured into a tubular form. The slide core is placed on the radially inner side of the coil and covers an outer peripheral portion of the plunger. The slide core axially slidably supports the plunger and conducts the magnetic flux between the slide core and the plunger in a radial direction. The magnetic shield portion limits flow of the magnetic flux between the magnetically attracting core and the slide core.
The stator core is inserted into an inside of the yoke from one axial side of the stator core where the slide core is located, and the stator core is fixed to the yoke at the opening of the yoke. A distal end of the slide core is inserted into a hole formed in the bottom wall portion of the yoke such that a predetermined size of an installation gap is formed between the bottom wall portion of the yoke and the distal end of the slide core, so that the distal end of the slide core forms a free end that is not fixed to the yoke.
In the linear solenoid of JP4569371B2 (corresponding to US2006/0243938A1), a ring core is axially installed between the coil assembly and the bottom wall portion of the yoke to limit a reduction in the amount of the magnetic flux between the slide core and the yoke, caused by the presence of the installation gap. Here, the ring core is installed such that the ring core covers an outer peripheral portion of the slide core and is slidable relative to the slide core. Also, the ring core conducts the magnetic flux between the ring core and the slide core in the radial direction. The ring core contacts the bottom wall portion of the yoke and conducts the magnetic flux between the ring core and the bottom wall portion of the yoke in the axial direction.
However, due to the installation of the ring core, an axial size of the linear solenoid of JP4569371B2 (corresponding to US2006/0243938A1) is disadvantageously increased. Therefore, in order to meet a demand of increasing the number of winding turns of the coil and a demand of reducing the axial size without deteriorating the advantage of enhancing the conduction of the magnetic flux with the ring core, additional measures are required.